Electrical circuits are known to include a plurality of circuit elements, wherein some of the circuit elements generate noise, while others are adversely affected by noise. Such noise generating circuits are generally digital type circuits such as transistors, random access memory (RAM) devices, read only memory (ROM) devices, line drivers, etc. Noise sensitive circuits are generally analog devices such as comparators, operational amplifiers, analog-to-digital converters, filters, bias circuits, voltage references, etc.
To limit noise interference between the noise generating circuits and the noise sensitive circuits, designers typically separate the "noisy" circuits (i.e. those that generate noise) from the noise sensitive circuits. To further reduce the noise interference, shielding may be placed around the noisiest circuits and/or around the noise sensitive circuits. The shielding is then coupled to a common line such as ground or a voltage source line.
This shielding technique has also been incorporated on circuit dies which are used in flip-chips, surface mount devices, and integrated circuits. Circuit die shielding improves noise isolation over non shielded techniques, but, when the noisy circuits, the noise sensitive circuits, the shielding, and the substrate all share the same common path on the circuit die, substrate noise still interferes with the performance of the noise sensitive circuit elements.
To further reduce noise interference on circuit dies, a first lead is provided on the circuit die for the substrate and another lead is provided for the shielding, the noisy circuit elements, and the noise sensitive circuit elements. The separate leads are connected, externally from the circuit die, to a common point such as system ground or a supply voltage. While this technique improves noise isolation over the single lead approach mentioned above, noise still affects the noise sensitive circuit elements. Therefore, a need exists for method and apparatus that further improves noise isolation on circuit dies.